This invention relates to waveform generators.
More particularly the invention relates to waveform generators suitable for use in the testing of instrument landing systems (ILSs).
An ILS is a system which is capable of guiding an aircraft in blind flying conditions to land. The system comprises ground based transmitters which transmit two pairs of beams which overlap to define a narrow pencil landing flight path, and an aircraft based receiver. One beam of each pair comprises a radio frequency (rf) carrier amplitude modulated with a frequency of 90 Hz. The other beam of each pair comprises an rf carrier amplitude modulated with a frequency of 150 Hz. In each pair of beams the depth of the 90 Hz amplitude modulation is the same as the depth of the 150 Hz amplitude modulation, i.e. the amplitude of the 90 Hz modulating waveform is the same as the amplitude of the 150 Hz modulating waveform. One pair of beams defines the verticle angle of approach (glide path). The other pair of beams defines the horizontal angle of approach (direction of approach).
If the aircraft is in the correct position with respect to both of the pairs of beams the receiver indicates in respect of each pair equal amounts of 90 Hz and 150 Hz modulation, i.e. indicates in respect of each pair that there is no difference in the depth of the 90 Hz and 150 Hz amplitude modulation (DDM). If the aircraft is not in the correct position with respect to either pair or both pairs of beams the receiver indicates a dominance in one frequency of modulation in one or each of the pairs of beams, i.e. indicates for one pair or for each pair that there is a DDM. The amount of the DDM for each pair indicates the extent to which the aircraft has moved off path. The system thereby guides the aircraft to land.
To test a receiver it is necessary to be able to generate a test waveform comprising the superposition of 90 Hz and 150 Hz waveforms of the same amplitude. It is also necessary to be able to generate test waveforms having various specified different amplitudes. In the testing, these waveforms are used to amplitude modulate the aforementioned rf carrier. The waveform wherein the 90 Hz and 150 Hz components are of the same amplitude can therefore be termed the zero DDM waveform, and the waveforms wherein the 90 Hz and 150 Hz components have different amplitudes termed non-zero DDM waveforms. It is important that the total amplitude is the same for all the test waveforms since in practice in the ILS when the aircraft moves off path although the DDM changes the total depth of modulation remains the same.
One prior art generator for generating such test waveforms comprises a direct digital synthesiser (DDS) comprising a digital accumulator, a PROM or RAM look-up table and a digital to analogue D/A converter. The look-up table stores 1/30 of a second of each of a number of alternatively selectable test waveforms to be provided (the zero DDM waveform and the non-zero DDM waveforms), it being sufficient to store 1/30 of a second of each since this is the repeating period of the test waveforms. The look-up table is addressed by the digital accumulator which determines the timing of the test waveform provided by the look-up table. The D/A converter converts into analogue form the selected test waveform provided by the look-up table.
A problem with this prior art generator is that for satisfactory operation for the generation of the zero DDM waveform and the various non-zero DDM waveforms very fine resolution and good linearity is required for the D/A converter. Further, it is inconvenient to store the many waveform in the look-up table.